L4 - measuring disease in populations Flashcards
Define incidence
The number of new cases of a disease that has occurred.
Define prevalence in words
The number of people affected by a disease at a specific time/ time frame. This counts people with existing disease as well as new cases. It however is not a rate but a proportion (denominator is persons, not persons per time).
Describe the inter-relationships between incidence and prevalence
P roughly equal (I x L) where L=length of disease, when the incidence rates and rates of recovery and death are constant.
Factors that increase prevalence in a population - increase incidence (new cases), keep them alive longer.
Factors that decrease prevalence in a population - increase death (death rates) and cures (cure rate)
Explain the purpose of age-sex standardisation
SMRs give a single summary measure of disease corrected for age-sex confounding. So you can compare two populations as IF their age-sex structure were the same.
How do you interpret a standardised mortality/morbidity ratio (SMR)?
100= same risk in the study population as in the standard reference population >100 = higher risk in the study population
Why is the systematic variation in the risk of disease between groups, a useful source of information?
Incidence is a measure of the population’s AVERAGE RISK of disease, but there are variations in risk of disease between groups of people.
SYSTEMATIC VARIATIONS IN RISK BETWEEN PEOPLE (compare incidence rates between groups with different levels of exposure) gives us clues about the CAUSE OF DISEASE.
Why is the systematic variation in the risk of disease between groups, sometimes a nuisance which needs to be controlled for?
Factors such as age and sex that systematically vary between groups are strong determinants of health but are not potential hazards that can be modified to target prevention. They can be confounding factors.
Why is measuring the incidence important?
It focuses on NEW EVENTS. Useful when monitoring epidemics or when monitoring the effect of prevention programmes. Or when a disease has a very short duration e.g. stroke.
Why is measuring the prevalence important?
It describe the burden of disease. It is a useful measure of the need for services.
Define the incidence rate using words
Incidence rate divides the new cases observed (events) by the number of person-years (number of people observed multiplied by number of years they are observed for)
Define the incidence rate using maths
incidence rate = (new events)/(person x time (years)) = events per person per year
Define (point) prevalence using maths
(point) prevalence = (number of cases (new and old) in a time frame)/ population
As time frame is not in the denominator it is NOT A RATE!
Define a cross-sectional survey
is a type of observational study that involves the analysis of data collected from a population, or a representative subset, at one specific point in time.
Define control group
A group of subjects or conditions that is matched as closely as possible with an experimental group, but is not exposed to any experimental treatment/conditions. The results are then compared to determine the changes that may occur due to the exposure.
Define incidence rate ratio (IRR) in words
Comparing the incidence rates between groups with different levels of exposure.